Improved conversion to CyTI types

Implement conversion from Python lists to TI-8x real list variables,
create a method to automatically convert a Python variable to the
appropriate CyTI type, and make some minor improvements to variable
creation/conversion in general (allow calculator model codes, convert
real variable theta).
Handle conversion of 0 to TI real

Ensure that 0 can be properly converted to a TI Real variable.
Improved list conversion handling

Ensure that lists have 999 or fewer elements when attempting to convert
them (otherwise, memory errors on calculators may result). Additionally,
ensure that numbered lists are in the correct range and automatically
convert lowercase list names.
Improved variable sending

Implement better automatic conversion of variables when sending and
allow the use of the array-style syntax.